As a secondary battery, a lithium ion battery using an electrolytic solution, a NaS battery, and the like have been known. These lithium ion batteries and NaS batteries have operable lower limit temperatures, for example, about 0° C. (at the time of charging) for the lower limit temperature of the lithium ion battery, and about 300° C. for the lower limit temperature of the NaS battery. Therefore, for example, when an air temperature is low, it is necessary to maintain the temperature of a secondary battery using a heater or the like such that it does not fall below the lower limit temperature.
In addition, an all-solid battery which has attracted attention in recent years has a wide operating temperature range such as being capable of charging and discharging the battery at about −30° C., but has an internal resistance rising at a low temperature like a conventional battery. Therefore, it is desirable to operate the all-solid battery at a high temperature (for example, about 150° C.) to fully exhibit its performance.
Patent Literature 1 discloses a technology that can heat a secondary battery without using a heater by using joule heat at the time of discharge of a NaS battery by performing charging and discharging between a plurality of NaS batteries constituting the secondary battery, that is, by performing control such that one NaS battery is discharged and the other NaS battery is charged.